We report the study of methanol electro-oxidation on ordered and defective Pt(1 1 0) surfaces using infra-red-visible sum-frequency generation vibrational spectroscopy in the CO ads internal stretch spectral region (1700-2150 cm À1). Comparison of the spectra as a function of the structural quality of the electrodes and of the applied potential shows that: (i) dissociative chemisorption occurs primarily at low potentials on defect sites such as steps and (ii) the signal of CO adsorbed on terraces decreases more sharply at higher potentials than the signal of CO adsorbed on defects. This latter behavior appears to contradict the fact that dense CO adlayers on terraces block the oxidation process. This apparent paradox is solved by considering the interplay of preferential oxidation of CO at defects sites and transfer of adsorbed CO from terraces reservoir to defects.